1. Field of the Invention
The present invention relates to a highly active homogeneous organic titanium catalyst used for selective hydrogenation of unsaturated double bonds in conjugated diene units of the living polymer, and a process for hydrogenating the polymer.
2. Description of the Prior Art
Polymers obtained by subjecting a conjugated diene to polymerization or copolymerization are widely used as an elastomer in industry. These polymers have the remaining unsaturated double bonds in the polymer chain, which are advantageously utilized in vulcanization on one hand, but render the polymers poor in stability such as weather resistance and oxidation resistance. Block copolymers obtained from conjugated diene and vinyl-substituted aromatic hydrocarbon are used in the unvulcanized state as a thermoplastic elastomer, a transparent impact-resistance resin or a modifier for styrene resins and olefin resins. However, these copolymers are inferior in weather resistance, oxidation resistance and ozone resistance because of the unsaturated double bonds remaining in the polymer chain, which makes the copolymers unsuitable for use in the field of outer-coating materials.
Generally, the improvement of weather resistance and oxidation resistance can be accomplished by hydrogenating the polymers to saturate the unsaturated double bonds remaining in the polymer chain. Many processes have been reported for hydrogenating polymers having unsaturated double bonds. In general, there are two known processes. Namely, a process using heterogeneous system catalyst and a process using homogeneous system; Ziegler type or organic complex catalyst, such as, an organometallic compound of Rh or Ti.
In the process using the heterogenous system, the hydrogenation reaction is caused by the contact of hydrogen with polymer on the heterogeneous catalyst. Therefore, it becomes difficult for the reactants to contact with the catalyst owing to the viscosity of the reaction system and steric hindrance of polymer chain. Hence, a large amount of the catalyst and more severe conditions are required for the efficient hydrogenation of polymer. As a result, it causes the decomposition and gelation of polymer. In the case of hydrogenation of the copolymer of conjugated diene and vinyl substituted aromatic hydrocarbon, the aromatic nucleus portions of the copolymer are usually hydrogenated, and it is difficult to selectively hydrogenate only the unsaturated double bond of the conjugated diene units. Furthermore, the physical removal of the catalyst after hydrogenation from the hydrogenated polymer is difficult and complete removal is substantially impossible.
On the other hand, in the process using the homogeneous catalyst system, the catalytic activity is high and the hydrogenation can be carried out under mild conditions. Therefore, it is possible to selectively hydrogenate the unsaturated double bonds of the conjugated diene units in the copolymer of conjugated diene and vinyl substituted aromatic hydrocarbon.
The hydrogenation or selective hydrogenation of conjugated diene polymers has been accomplished using any of the several hydrogenation processes known in the prior art, for example, U.S. Pat. Nos. 3,494,942; 3,670,054; 3,700,633. These methods known in the prior art for hydrogenating polymers containing ethylene unsaturation and for hydrogenating or selectively hydrogenating polymers containing aromatic and ethylene unsaturation involve the use of a suitable catalyst, particularly, a catalyst or catalyst precusor comprising a Group VIII metal.
In the methods described in foregoing patents, a catalyst is prepared by combining a Group VIII metal, particularly nickel or cobalt, compound with a suitable reducing agent such as aluminum alkyl. Also, while aluminum alkyls are the preferred reducing agents, it is known in the prior art that alkyls and hybrids of metals of Groups I-A, II-A and III-B of the Periodic Table of the Elements are effetive reducing agents, particularly lithium, magnesium and aluminum. In general, the Group VIII metal compound is combined with Group I-A, II-A or III-B metal alkyl or hybride at a concentration sufficient to provide Group I-A, II-A and/or III-B metal to Group VIII metal ratio within the range from about 0.1:1 to about 20:1, preferably from about 1:1 to about 10:1.
U.S. Pat. No. 4,501,857 disclosed that selective hydrogenation of the unsaturated double bonds in conjugated diolefin polymers could be achieved by hydrogenating such polymers in the presence of at least one bis(cyclopentadienyl)-titanium compound and at least one hydrocarbon lithium compound. Also, U.S. Pat. No. 4,980,421 disclosed that similar hydrogenation can be accomplished with the same titanium compounds in combination with an alkoxy lithium compound which can either be added directly or as a reaction mixture of an organo lithium compound with an alcoholic or phenolic compound. The use of these catalyst systems was said to be advantageous, because the catalysts were said to be highly active so that they were effective even in such a small amount as not to affect adversely the stability of a hydrogenated polymer and require no deashing step.
In U.S. Pat No. 4,673,714, bis(cyclopentadienyl)-titanium compounds were disclosed which preferentially hydrogenate the unsaturated double bonds of conjugated diolefins but do not require the use of an alkyl lithium compound. These titanium compounds were bis(cyclopentadienyl) titanium diaryl compounds. The elimination of the need for the hydrocarbon lithium compound was said to be a significant advantage of this invention.
In U.S. Pat. No. 5,039,755, the process for hydrogenation of conjugated diolefin polymer which comprises polymerizing or copolymerizing at least one conjugated diolefin with an organo alkali metal polymerization initiator in a suitable solvent thereby creating a living polymer. The living polymer was terminated by the addition of hydrogen. The selective hydrogenation was carried out in the presence of at least one bis(cyclopentadienyl) titanium compound preferably of the formular (C.sub.5 H.sub.5)Ti R.sub.1 R.sub.2.
However, in the case of the homogenous catalyst, the hydrogenation activity is greatly varied depending upon the reduction state of catalysts, and therefore, the reproducibility of hydrogenation is inferior and it is difficult to obtain the hydrogenated polymer having a high degree of hydrogenation with a high reproducibility. Moreover, the catalyst components tend to be converted into inactive substance owing to impurities. Therfore, the impurities decline the hydrogenation activity and reproducibility. The fact that highly hydrogenated polymer cannot be obtained due to its low reproducibility is a great obstacle to utilize the homogeneous catalytic hydrogenation for the purpose of enhancing the weather resistance and heat resistance of the polymers. Further, in the conventional hydrogenation of polymers with homogeneous catalyst, the hydrogenation rate cannot be said to be sufficiently high. Therefore, it has been a problem in hydrogenating the polymer with a homogenous catalyst in industry. With these reasons, it has been strongly desired that there is developed a highly active hydrogenation catalyst which is capable of producing the highly hydrogenated polymer at a high hydrogenation rate stably.